Various types of wind machines including constant rotor speed and adjustable speed machines are known. Since the turbine-generator is normally directly connected to a fixed frequency electrical power grid constant speed wind turbines have predominated. Variable speed wind turbines cannot be directly connected to the grid because of their variable frequency electrical power output. This necessitates the interposition of a frequency converter between the generator and the grid to make the frequency conversion. Various techniques are well known for such frequency converters including cycloconverters and rectifier-inverters.
In addition, electronic speed control techniques have been developed for all types of motors using the thyristor, or silicon-controlled rectifier (SCR). It has become apparent to wind energy theorists that it is possible to use an adjustable speed AC drive in reverse. In other words, instead of using the adjustable speed drive as a receiver of fixed frequency AC for conversion to variable frequency AC for driving a motor, it is possible to provide the frequency converter with variable frequency AC from an AC generator for conversion to fixed frequency AC for supplying a power grid.
As known in the wind energy art, the kinetic energy available from an oncoming wind stream varies as the size of swept area, density, and cube of the wind velocity. It has been shown that no more than 59% of the energy can be extracted and the ability of any wind turbine to approach that maximum has been named the coefficient of performance, C.sub.p. Coefficient of performance is related to the aerodynamic features of a given machine, particularly the tip speed ratio, which is defined as the ratio of tangential speed of the blade tip over the speed of the oncoming wind. If this ratio can be maintained at the machine's peak coefficient of performance by letting rotor speed follow wind speed, the wind turbine becomes highly efficient. In addition, variable speed wind turbines provide the opportunity for short term energy storage. i.e., wind speed transients can be integrated into speed changes. The control strategy for such a wind turbine should be based on electrical adjustment of generator torque.